A. antecedens is known only from a single fragmentary skeleton including shoulder, wing, and leg bones, but lacking a skull. It was about the size of a sparrow.[3] The specimen was discovered in 1971 and described and named in 1976 by Pierce Brodkorb. Brodkorb considered the species to be similar to some modern birds, and at the time, recognized it as one of the only known Cretaceous "land birds" after Gobipteryx minuta (most other Cretaceous birds at the time were thought to be aquatic or semi-aquatic).[2]

Alexornis was originally described as an early member of the modern bird group and, more specifically, as a possible ancestor of the ordersCoraciiformes (kingfishers and relatives) and Piciformes (woodpeckers and relatives). However, when the first recognized enantiornithine fossils were discovered, it soon became clear that Alexornis was an additional member of this group.[3] Enantiornithes are characterized by a reversed articulation of the scapula and coracoid bones in the shoulder, and in 1983 Larry Martin showed that Brodkorb has accidentally switched these two bones in his original description, confused by their reversed anatomy.[4]

In the past, Alexornis has been allied with the similarly-sized Central Asian genus Kizylkumavis, based on characteristics which are known to unite them with other Cretaceous birds in the group Enantornithes.[3]Evgeny Kurochkin argued in 1996 that Alexornis formed a natural group, the Alexornithidae, with Kizylkumavis and a second type of central Asian bird, Sazavis. Among the enantiornithes, Alexornis shares many similarities of its skeletal anatomy with South American birds from about the same time period, including Martinavis and Elbretornis, and is united with them in the group Euenantiornithes.[5]

An Alexornis named "Alex" was included in the 2013 animated film Walking with Dinosaurs as a primary character and narrator, voiced by actor John Leguizamo. The character was given a Mexican accent to reflect the geographic origin of the bird despite the film being set in Alaska.[6] In an interview, Leguizamo told the Web site Gozamos, "this Alexornis bird (Alex) was just found in Mexico, so we gave it this Spanish flavor."[7] Due to the fragmentary nature of the fossils, the actual appearance of the Alexornis in the film was based on related species.[8]